This invention concerns, in general, ovens which heat by infrared radiation, and, more specifically, ovens designed to heat preforms made of thermoplastic materials for the manufacture of hollow bodies such as bottles, flasks, or similar objects by blow molding or stretch blow molding.
Conventional machines used to manufacture these hollow bodies by blow molding or stretch blow molding; incorporate at least a preform thermal treatment station, in which the preforms are heated to a temperature such that they can then be molded in a blow molding or stretch blow molding; operation and a station for blowing the preforms in molds whose impressions have the shape of the hollow bodies to be produced. Respective conveyors provide for the feed of the preforms to the thermal treatment station and the transfer of the heated preforms between the treatment station and the blowing station.
Heating by infrared radiation is conventionally known and widely used in the industry for the thermal treatment of plastic preforms, in particular those made of polyethylene terephthalate (PET), the preforms being designed for the manufacture of hollow bodies such as bottles. In comparison with other heating or thermal treatment methods such as convection and conduction, and considering the low level of thermal conductivity of the material, heating using infrared radiation gives advantageous output and allows increased production rates.
Given the high energy density given off in these ovens, it is necessary to cool the parts of the oven most exposed to infrared radiation, in particular the stationary parts, in order not to damage them. To this end, a flow of cooling air which carries away excess calories has become necessary. In conventional installations, air is collected in the premises on which the oven is installed, using one or more fans. Once thermal exchange has been achieved, this air is evacuated within the premises at the top part of the oven.
The search for ever-increasing rates of production, the blowing of bottles or containers having increasingly complex shapes, and reducing the weight of the preforms while preserving, and indeed enhancing, the technical performance of the finished articles, require a distribution of material in the final product which is both precise and accurately repeated during the entire production run. These requirements make it necessary to control completely the thermal treatment of the preforms, thereby requiring precise adjustments regulating the heating profile. A very high degree of precision is thus made mandatory, not only in the installation of the infrared emitting sources and the associated intensity controls, but also in the temperature stability of the air flow circulating through the oven.
Tests have demonstrated the harmful effect of any temperature variation of this air flow, in particular on thickness distribution and, more generally, on the quality of the final article. This air, which is drawn from the shop where the machine is installed, such shop not normally being equipped with an expensive airconditioning unit, undergoes temperature changes from summer to winter, from day to day, and as a function of parameters such as the opening of a door, for example. In fact, it appears that, during the course of one day, temperature variations may be significant. Since the regulation circuit adjusting the thermal preform processing is controlled by an infrared pyrometer which measures the temperature of the outer walls of the preforms, any temperature variation of the flow of cooling air will affect the temperature of outer preform walls. The relationship obtaining between the quantity of heat stored in the preforms and the reading of the infrared pyrometer will then be distorted. As a result, the regulation of the thermal treatment is disrupted and the exact adjustments of the preform heating profiles are no longer correct, thereby leading to the manufacture of hollow bodies of disparate quality.